A ventricular assist device, or VAD, is a mechanical device used to help either one or both ventricles of the heart to pump blood. VADs are designed to assist either the right (RVAD) or left (LVAD) ventricle, or both at once (BVAD). The choice of device depends on the underlying heart condition, although LVADs are the most widely used.
In early procedures, the VAD pump was placed outside the body. More recently, the most commonly used VADs comprise an integrated pump and are implanted during open or closed-heart surgery, with a control line passing through the skin to a body worn controller. The use of implanted VADs with integrated pumps improves patient mobility and therefore a patient's capacity to lead a near normal life outside hospital.
Some VADs make use of continuous-flow impeller pumps whose only moving part is a rotor. These are relatively small, easy to insert, and are expected to be reasonably durable. They are considered capable of maintaining adequate circulation, although their capability to fully unload the ventricle is questionable. Furthermore, the use of these pumps requires full anti-coagulation therapy coupled with antiplatelet medication and there is a risk of bleeding and/or thromboembolic complications partly caused by the very small clearance of the rotor inside the device.
Other VADs make use of centrifugal pumps in which the rotor is magnetically or mechanically suspended and, therefore, does not use ball bearings. This feature, coupled with the lower number of revolutions per minute, should provide enhanced durability. However, such VADs may also present a risk of complications including thromboembolic complications.
Up until recently, the most commonly used VADs however are pulsatile devices that mimic the natural pulsing action of the heart. These pulsatile devices use positive displacement pumps with pusher plates and inflow and outflow valves. These devices are efficient at unloading the ventricle which could help recovery of the native heart while maintaining circulation. Pulsatile devices do, however, have several disadvantages, such as their large size and complexity, which can increase the risk associated with insertion, predispose the patient to infection, and compress adjacent organs. They may also contain many moving parts that can affect their durability.
FIG. 1 illustrates a pulsatile type LVAD in situ. An inflow tube 1 is inserted into the apex of the left ventricle. Upon contraction of the ventricle, blood passes through the inflow tube 1 to a pump within the device housing 2 and then out of the pump through an outflow tube 3 to the aorta. One-way valves are associated with the inflow and outflow tubes to prevent blood from flowing back from the aorta into the housing 2, or from the housing 2 to the ventricle, during the pumping action of the device. A lead 4 extends from the device, through the patient's skin, connecting the device to a power supply and to a control computer, both worn externally by the patient.
It will be appreciated that the LVAD of FIG. 1 receives a flow of blood from the left ventricle, traps this, and pushes it out to the aorta. This requires a relatively powerful pump comprising large pusher plates. This in turn requires venting of the outer sides of the pusher plates to the open air, requiring a venting tube 5 passing through the skin. Without this venting the displacement pumps would consume excessive power, as they would have to displace the pusher plates against a vacuum.
Whilst conventional LVADs can provide significant therapeutic benefits, they may also give rise to complications including infection, immunosuppression, clotting with resultant stroke, and bleeding secondary to anticoagulation. By way of example, there is a high risk of the formation of blood clots within an LVAD in regions where blood flow is stagnant and this in turn requires the use of anticoagulation therapy in order to prevent thrombosis (clotting). The use of anti-coagulants then leads to an increased risk of bleeding.
The design of existing commonly used VADs suffers from a number of other weaknesses. The implantable devices are generally large (in the region of 120 cc in volume) and heavy and usually require open-heart surgery for implantation with the associated risks. Their large size also prevents these devices from being placed within the chest cavity, due to lack of space, so that they are usually positioned in the stomach area, making it necessary to use a more powerful pump given the increased distance over which the blood is required to travel.
Evidence has shown that the use of LVADs, in conjunction with an appropriate drug therapy, can potentially lead to recovery of the patient without the need for further, more drastic treatment. It would be desirable to use LVADs in a greater number of patients. However, the risks associated with open heart surgery, as required to implant the existing LVADs, and with other complications is too great for those patients with less severe heart conditions. In addition, the size of existing devices mean that they are only suitable for use in patients weighing more than around 70 kg, preventing their use in small adults or children. A smaller and more reliable LVAD that could be implanted using less invasive techniques would likely increase the use of LVADs.
WO 2000/076288 discloses a different approach to assisting the heart and makes use of an inflatable cuff around the aorta. Inflating the cuff contracts the aorta and deflating the cuff allows the aorta to expand—in effect the aorta becomes a second left ventricle. The device described has potential advantages in avoiding the need to operate on the heart itself and in avoiding any contact between blood and the device. However this method poses a potentially significant risk of damage to the tissue of the aorta and histological changes have been observed in the outer wall of the aorta during animal and early clinical trials of such a device.